Just a guess, but it looks like a gas bubble has formed in its digestive tract. Possible causes would be impaction from to much protein, eating a piece of sand/gravel that is to big to pass easily, or clogged with parasites.
I would try feeding him thawed frozen peas. It's trick I've used over the decades with goldfish, and cichlids when they have buoyancy issues. It acts like a laxative for the fish. Doesn't always work, but anything is worth a shot.